2OO Paradigm - Review Three Characteristics of OO Languages Inheritance It isn’t necessary to build every class from scratch – attributes can be derived from other classesPolymorphism/virtual functionsObjects that belong to subclasses of the same superclass can be treated as if they are all objects of the superclass.EncapsulationObject behavior and object data are combined in a single entity. Object interface defines interaction with the object; no need to know/understand the implementation (and in general no way to access the implementation.)
3PolymorphismPolymorphism is a programming language feature that allows values of different data types to be handled using a uniform interface.3/29/2010
4Object Orientation in Python In Python, everything is an object – integers, strings, dictionaries, …Class objects are instantiated from user- defined classes, other objects are from language defined types.
5Python Classes Can be defined anywhere in the program All methods and instance variables are public, by defaultThe language provides “limited support” for private identifiers (see section 9.6 in the documentation).
6Exampleclass MyClass:def set(self, value):self.value = valuedef display(self):print(self.value)MyClass has two methods: set and display; and one attribute: value.The class definition is terminated by a blank line.
7ExampleThe first parameter in each method refers to the object itself. Self is the traditional name of the parameter, but it can be anything. def set(self, value): self.value = value When the method is called, the self parameter is omitted
8Python Class Objects y = MyClass() There is no new operator; to instantiate an object for a class, use function call notation: x = MyClass()y = MyClass()Each time a class is called, a new instance object is created. If the class has a constructor, you can use it here.
9Example Declare and assign value to a class variable: >>> y = MyClass()#”declare” a MyClass object>>> y.set(4) # no param corresponding to>>> y.display() # “self” in either call4>>> y.value
10Constructor - ExampleConstructors are defined with the __init__ method. Instead of def set(self, value): use def __init__(self, value):
11Constructorsdef __init__(self): self.value = 0 # default or def __init__(self, thing): self.value = thing # parameterized Usage: x = MyClass( ) sets x.value to 0 y = MyClass(5) sets y.value to 5 (default constructor and parameterized constructor)
12Class with Constructor >>> class Complex: def __init__(self, realp, imagp): self.r = realp self.i = imagp >>> x = Complex(3.0, -4.5) >>> x.r, x.i (3.0, -4.5)
13Attributes (Data Members) Attributes are defined by an assignment statement, just as variables are defined (as opposed to being declared).def set(self, value):self.value = valueCan be defined in classes or instances of classes.Attributes attached to classes belong to all subclasses and instance objects, but attributes attached to instances only belong to that instance.
14Python Class Data Members Variables in Python are created when they are assigned to.New data members (attributes) can be created the same way; e.g., x.counter = 1 creates a new data attribute for the object x – but not for y.
15Example (from online documentation) Variables in Python are created when they are assigned toNew data attributes can be created the same way:. . .x.counter = 1while x.counter < 10:x.counter = x.counter * 2print(x.counter)del x.counterx.counter = 1 creates a new data attribute for the object x – but not for y.
16Information HidingThere is no foolproof way to enforce information hiding in Python, so there is no way to define a true abstract data typeEverything is public by default; it is possible to partially circumvent the methods for defining private attributes.
17Inheritanceclass DerivedClassName(BaseClassName): <statement-1> <statement-N>If a requested attribute is not found in the derived class, it is searched for in the base class. This rule is applied recursively if the base class itself is derived from some other class.
19Multiple InheritanceResolving conflicts: (Given the expression object.attribute, find definition of attribute)Depth-first, left-to-right search of the base classes.“Depth-first”: start at a leaf node in the inheritance hierarchy (the object); search the object first, its class next, superclasses in L-R order as listed in the definition.
20Illustration of Single & Multiple Inheritance B1.a.bB2.xB3.y.bSUPERCLASSESB4.x.qDERIVED CLASSI0.xI1I2INSTANCESI0.a and I0.b are defined in B1; I0.x is defined in I0I1.a, I1.b and I2.a, I2.b are defined in B1; I1.x & I2.x (and q) are defined in B4I1.y and I2.y are defined in B3, and so forth.
21Python and OO Programming See Section 9 in the Python tutorial for more information and examples. Chapter 13, section 5 has several examples of Python classes.